Clarify multiple agent documentation

[jputrino]

Agent Version

8.3.0

OpenStack Release

Liberty

Description

Per customer SR#1-3137337841 -- the multiple agent documentation states

"We refer to ‘hosts’ a lot in this document. A ‘host’ could be a Neutron controller, a compute node, a container, etc.; the important takeaway is that in order to run multiple agents in one environment, each agent must have a unique hostname."

This is unclear and misleading and can cause confusion.
Change 'each agent must have a unique hostname' to 'each agent must have a unique agent_id'.